The invention concerns a method according to the technique of a Steady State Free Precession (SSFP) gradient echo method, in particular, of nuclear magnetic resonance (NMR) tomography, wherein a regular sequence of radio frequency pulses with flip angle α is applied at constant time intervals TR, wherein the phase of these pulses is increased in successive steps by a constant phase increment.
SSFP Imaging
An SSFP signal is generated by a continuous sequence of radio frequency pulses and was introduced by Carr already in 1958 [1]. Carr was able to show that implementation of the method with equidistant radio frequency pulses with constant amplitude and alternating phase produces an SSFP signal of on-resonance spins with particularly high signal intensity.
In 1986, this principle was transferred to an MR imaging method in the form of the FISP method (today called TrueFISP) [2]. All gradients are switched in such a manner that their integral from the center of a pulse to the center of the next pulse is zero. Successive pulses have flip angles α and alternating phases: P1, P3, P5 . . . =α, P2, P4, P6 . . . =−α. The time distance between two pulses is called repetition time TR (see FIG. 2).
One problem with its implementation is the fact that the incrementation of phase encoding gradients which is required for imaging can produce temporally variable eddy current effects and hence signal fluctuations. In particular, in SSFP applications with non-linear data acquisition (k space scanning) and consequently large amplitude jumps of neighboring phase encoding steps, this sensitivity of SSFP imaging to eddy current effects produces strong artefacts in the MRT image (FIG. 1) [3, 4].
Eddy Current Suppression
These signal fluctuations which are induced by eddy currents can be suppressed by adjusting the k space scanning in such a manner that a directly neighboring phase encoding step is read-out prior to each major k space jump (“paired phase encoding”) [5].
The suppression of signal modulation can be explained by the fact that the SSFP signal is determined by the production of a dual steady state configuration. A certain arrangement of the signal-producing magnetization is thereby mapped to each other in successive RF excitations. By changing the gradient amplitudes for phase encoding in successive RF excitations, the MRT signal phase induced by eddy currents is changed, which can disturb said mapping of the dual steady state magnetization configurations to each other. This configuration disturbance results in termination of the SSFP steady state, thereby causing signal fluctuations and image artefacts.
This formation of image artefacts and disturbance of the dual steady state configuration with the use of incremental phase encoding is also shown in FIG. 3B. FIG. 3B shows the temporal development of the transverse component of the MRT magnetization vector in the steady state for two successive data acquisition intervals ((1)->(2) and (3)->(4)) and RF excitations with flip angles α(rf(+α) and rf(−α)). The temporal development of the MRT signal and associated potential signal oscillations are determined by the temporal development of position and absolute value of the magnetization vector. The transverse magnetization is not mapped to itself due to different phase encoding steps and therefore different signal phases (Δφeddy,I and Δφeddy,II) induced by eddy currents after two data acquisition intervals and RF excitations ((4)->(1′) instead of (1)). The dual configuration of the SSFP steady state is thereby disturbed, producing signal fluctuations and image artefacts.
In the “paired phase encoding” acquisition strategy, phase encoding gradient differences are minimized in pairs to ensure that there are none or only minimum differences in pairs in the signal phases induced by the eddy currents. As a result, any eddy current phase can be compensated for either completely or partially during the subsequent excitation.
Data recording with “paired phase encoding” therefore reduces the eddy current artefacts but permits no complete compensation since the pairwise successive phase encoding steps still have a small gradient amplitude difference and can therefore produce different signal phases induced by eddy currents (see also FIG. 3B).
The best results are obtained with so-called on-resonance spins which do not experience any additional phase changes during TR. It has turned out, however, that the eddy current sensitivity, i.e. the incomplete compensation of eddy current effects, of the method increases with the local off-resonance frequency, i.e. the field inhomogeneity, in the tissue to be examined.
In MRT applications, this condition is, however, not met even for very small repetition times TR, wherein the shortest achievable TR is substantially determined by the switching speeds of the magnetic field gradients. Due to the magnetic field inhomogeneities, the spins are dephased to a certain degree by a phase angle Δφ between two excitations. For TR=4 ms, Δφ=90° for an off-resonance frequency ΔΩ of Δφ/(TR*360°)=66 Hz. This corresponds to an inhomogeneity of 1 ppm for a resonance frequency of 63 MHz with 1.5 tesla field strength. These inhomogeneities cannot be prevented in applications on human beings due to the arising susceptibility effects and local variations of the chemical shift.
Further MR methods which are relevant for the inventive method, relate to the acceleration of MRT imaging with parallel imaging [6, 7] and on the interruption of the SSFP steady state using “Steady State Storage” [8]. The latter is a method which permits insertion of preparation sequences (e.g. for fat saturation) into an SSFP measurement in such a manner that artefacts due to interruption of the steady state are reduced. However, the best results with this method are also achieved for so-called on-resonance spins, whereby the artefact sensitivity, i.e. non-ideal “Steady State Storage” increases with the local off-resonance, i.e. with the field inhomogeneity in the tissue to be examined.
It is the underlying purpose of the invention to propose a method which minimizes the artefacts produced through incrementation of phase encoding gradients.
Optimized eddy current compensation (N-average SSFP imaging)